Sliding thermal contact for pluggable optic modules

ABSTRACT

Present thermal solutions to conduct heat from pluggable optical modules into heat sinks use a metal heat sink attached with a spring clip. The interface between the pluggable module and the heat sink is simple metal-on-metal contact, which is inherently a poor thermal interface and limits heat dissipation from the optical module. Heat dissipation from pluggable optical modules is enhanced by the application of thermally conductive fibers, such as an advanced carbon nanotube velvet. The solution improves heat dissipation while preserving the removable nature of the optical modules.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Patent Application No.61/817,382 filed Apr. 30, 2013, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an optical module, and in particular toa sliding heat sink for a pluggable optical module.

BACKGROUND OF THE INVENTION

Conventionally, optical transceivers with data rates up to 4 Gb/s arepackaged in small form factor (SFF or SFP) packages, while opticaltransceivers with higher data rates, e.g. 10 Gb/s, are in largerpackages, such as XFP, X2, and XENPAK. A conventional XFP arrangement isillustrated in FIG. 1, in which an XFP transceiver module 1 is pluggedinto a host cage assembly 2 mounted on a host circuit board 3. The hostcage assembly 2 includes a front bezel 4, a cage receptacle 5, and ahost electrical connector 6. The transceiver module 1 is insertedthrough an opening in the front bezel 4, and through an open front ofthe cage receptacle 5, until an electrical connector on the transceivermodule 1 engages the host electrical connector 6. The cage receptacle 5has an opening 7 in the upper wall thereof through which a heat sink 8extends into contact with the transceiver module 1 for dissipating heattherefrom. A clip 9 is provided for securing the heat sink 8 to the cagereceptacle 5 and thereby into contact with the transceiver module 1.With this arrangement, the heat sink 8 can be changed to suit theowner's individual needs without changing the basic transceiver module1.

Examples of conventional heat sinks are disclosed in U.S. Pat. No.6,916,122 issued Jul. 12, 2005 in the name of Branch et al.

Pluggable optic module thermal dissipation requirements are increasingwith the continued advancement of features and performance. 10 Gb/smodules with added features, e.g. EDC, tenability etc., have increasedthe power density of pluggable optics, and speed increases to 40 Gb/sand 100 Gb/s are pushing power densities even higher. A fundamentalproblem for all pluggable (removable) optical modules in telecom systemsis that the need to make them removable limits the thermal conductionpath. Improvements to the thermal conduction path will reduce the needfor faster cooling air speeds or larger heat sinks, which are not alwayscapable of keeping the modules within the operating temperature rangesspecified.

The most common approach to connecting a heat sink to a pluggableoptical module is the use of the MSA-suggested heat sink 8, which clipsto the cage 2 using the spring clip 9. The spring clip 9 enables theheat sink 8 to move slightly, i.e. up and down, side to side, forwardsand back, when the pluggable optic module 1 is inserted/extracted, whilemaintaining a tight interface between the surface of the module 1 andthe heat sink 8. However, the surfaces of the heat sink 8 and thepluggable optic module 1 are made of hard, non-conforming metal. Thismetal-to-metal contact is the weak link in the thermal path. Microscopicimperfections in the heat sink 8 and surfaces on the module 1 limit theflow of heat across the interface. Thermal contact resistance causeslarge temperature drops at the interfaces, which negatively affect thethermal performance of the system. Thermal management can besignificantly better if there are no high resistance interfaces in thesystem.

In non-sliding applications a thermal interface material, e.g. gel, isoften used to improve the thermal interfaces by filling theimperfections and improving heat flow. However, in a slidingapplication, e.g. pluggable optics modules (SFP, SFP+, GBIC, XFP,XENPAK, XPAK, X2) traditional thermal interface materials areundesirable because the thermal interface for pluggable optics istransient in nature. Modules will be extracted and inserted multipletimes. Thermal interface materials leave residue on modules as they areremoved, they dry out when no module is present (shipping) and aregenerally awkward to apply.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing heat-sinking pluggable optical modules whichaddresses the need to be able to insert and remove MSA standard or otheroptical modules. The solution provides greatly improved thermalconductivity between the optical module and the heat sink within thesystem.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a cage assembly mountableon a printed circuit board for receiving an optical module comprising:

a cage for slidably receiving the optical module;

an electrical connector mountable on the printed circuit board forelectrically connecting the optical module to the printed circuit board;and

a heat sink assembly mounted on the cage for dissipating heat from theoptical module, the heat sink assembly comprising:

a thermally conductive heat sink separated from the optical module by agap; and

a first thermal interface mounted on an underside of the heat sink,including thermally conductive fibers extending across the gap into thecage for contacting the optical module.

Another aspect of the present invention relates to an optical module forsliding into a cage assembly, which includes a cage, a first electricalconnector with an opening in an upper wall, and a heat sink assemblymounted on the cage over the opening, comprising:

a housing defining a gap with the heat sink assembly when inserted inthe cage;

optical and electrical components disposed in the housing for convertingoptical signals into electrical signals and electrical signals intooptical signals;

a second electrical connector extending from the housing for connectionto the first electrical connector;

an optical connector extending from the housing; and

a second thermal interface mounted on the housing including thermallyconductive fibers for extending through the opening and across the gapinto contact with the heat sink assembly for dissipating heat from thehousing.

Another feature of the present invention provides an optical systemincluding:

the aforementioned cage assembly; and

the aforementioned optical module;

wherein the thermally conductive fibers from each of the first andsecond thermal interfaces have a length between 0.6× and 1.0× a width ofthe gap between the optical module and the heat sink for engaging eachother.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 is an exploded view of a conventional optical module cage system;

FIG. 2 is an exploded view of an optical module cage system inaccordance with the present invention;

FIG. 3 is a cross-section view of the optical module cage system of FIG.2;

FIG. 4 is an isometric view of optical module;

FIG. 5 is a perspective view of a heat dissipating velvet of the opticalmodule cage system of FIG. 2;

FIG. 6 is a isometric view of a second embodiment of the presentinvention in which a single heat sink is utilized for a plurality ofoptical module cage systems;

FIG. 7 is an exploded view of the optical module cage system of FIG. 6;

FIG. 8 is an isometric view of a third embodiment of the presentinvention in which the velvet is mounted on the optical module; and

FIG. 9 is an isometric view of a fourth embodiment of the presentinvention in which velvets are mounted on both the optical module andthe heat sink.

DETAILED DESCRIPTION

With reference to FIGS. 2 and 3, the present invention relates to a cageassembly 12 for receiving a pluggable optical module 11. The cageassembly 12 includes a rectangular, metal cage 13, as is known in theprior art, mounted on a printed circuit board 15, as in FIG. 1. The cage13 includes a first opening 14 in a front wall for receiving thepluggable optical module 11, and a second opening 16 in an upper wallfor receiving a heat sink assembly 17. The second opening 16 is at leasthalf of the area of the upper wall, and preferably at least ¾ of thearea of the upper wall, e.g. up to 90% of the area of the upper wall. Anelectrical connector 18 is mounted in the cage 13 on the printed circuitboard for receiving a mating electrical connector on the pluggableoptical module. The printed circuit board 15 includes trace electricalconnectors for electrically connecting the connector 18 to a hostcomputer system, within which the printed circuit board 15 is received.

The optical module, e.g. SFP, SFP+, GBIC, XFP, XENPAK, XPAK, X2, CFP,CFP2, CFP4, or QSFP transceiver, generally indicated at 11 in FIG. 4,typically includes a ROSA 21 mounted in a housing 22 alongside a TOSA23. A PCB 24 includes TOSA and ROSA control and monitoring circuitry,e.g. chip 25. An electrical connector 27 extends from a rear end of thehousing 22 for mating with a host mounted electrical connector 6. For apluggable transceiver the electrical connector 27 includes a card edgeconnector formed in the end of the PCB 24. Bores 33 and 34 form anoptical connector on a front end of the housing 22 for receiving anduplex optical connector. Other types of electro/optical modules arepossible.

The heat sink assembly 17 includes any conventional heat sink 41,comprised of metal or other suitable thermally conductive material,preferably with a plurality of thermally conductive fins or fingersextending upwardly therefrom, enabling cooling air to pass over, aroundand between. The heat sink assembly 17 also includes a first slidingthermal interface 42 a in the form of a velvet or brush comprised of aplurality of thermally conductive whiskers, filaments or fibers disposedbetween the housing of the optical module 11 and the heat sink 41,whereby the whiskers, filaments or fibers extend through the secondopening 16 and across gap 19 between the optical module 11 and the heatsink 41. In an alternate embodiment a second sliding thermal interface42 b is mounted on the optical module 11, in place of or in conjunctionwith the first sliding thermal interface 42 a, whereby the whiskers,filaments or fibers extend upwardly from the optical module 11 throughthe second opening 16 into contact with the heat sink assembly 17, i.e.the first sliding interface 42 a or all the way to the heat sink 41, ifthe first sliding interface 42 a is absent.

Ideally, the heat sink assembly 17 covers the entire area of the secondopening 16, and the first (or second) sliding thermal interfaces 42 aand/or 42 b covers at least 50% of the second opening 16, preferably atleast 75% and more preferably up to 90%. Typically, each fiber isbetween 3 and 12 um in diameter, with a packing density of from 0.1% to24%, preferably 3% to 15%, and more preferably 4% to 6%. Typically, thevelvet 42 a and/or 42 b has a thermal conductivity greater than 500W/m²K, preferably between 1000 and 10,000 W/m²K, and more preferablyabout 2000 to 5000 W/m²K. Ideally, carbon nanotubes (FIG. 5) are used,which provide excellent thermal conductivity while maintainingmechanical compliance. Examples of carbon nanotubes are found in U.S.Pat. No. 7,416,019 issued Aug. 26, 2008 in the name of Osiander et al,and U.S. Pat. No. 8,220,530 issued Jul. 17, 2012 in the name of Cola etal, which are incorporated herein by reference.

With reference to FIG. 5, ideally, the “velvet” 42 a and/or 42 b iscomprised of carbon nanotubes in the form of a foil substrate 43 with anarray of carbon nanotubes 44. The preferred embodiment uses aspecifically designed carbon nanotube velvet to connect the pluggableoptical module 11 to a heat sink 41. The many fibers in the velvet 42 aand/or 42 b can move independently to fill the voids in the surfaces ofthe pluggable optic module 11 to improve the heat flow therebetween. Theindependent and flexible nature of the fibers also enables the surfacesto slide while still maintaining thermal contact. The improved contactlowers the temperature of the pluggable module 11 more than the standardmetal-on-metal contact of the MSA-specified heat sink design shown inFIG. 1.

In the primary embodiment of the invention, the velvet 42 a is mountedon the heat sink 41 of the cage system 12 into which the pluggable opticmodule 11 is being inserted, In this particular application, the carbonnanotube array 44 can be a velvet called VEL-THERM® procured from ESLI(Energy Science Laboratories, Inc.) disclosed in U.S. Pat. No. 7,132,161issued Nov. 7, 2006 to Knowles et al, which is incorporated herein byreference. The velvet 42 must be precut (die cut) to the precise sizerequired to extend through the second opening 16 in the optical modulecage 13. The thickness of the velvet 42 a or 42 b is preciselycontrolled to provide optimal contact with the pluggable optic 11 foroptimization of both thermal performance and the insertion and removalof the module 11. Typically, the thickness of the velvet 42 a or 42 b islarger than the gap 19, e.g. 1.2 mm, between the module 11 and the heatsink 41. Preferably, the thickness of the velvet 42 a or 42 b is between1.5× and 2.0× the width of the gap 19, e.g. 1.8 mm to 2.4 mm, andideally 1⅔× the width of the gap 19, e.g. 2 mm.

Another important consideration is the control of stray carbonnanotubes. Every effort is made to ensure that the pre-cut velvet 42 aand/or 42 b have no loose carbon nanotube fibers, which could dislodgeand interfere with the electrical operation of the circuit board 15 onwhich the optical module 11 is placed. An additional precaution is theapplication of an electrically insulating coating to the velvet 42 aand/or 42 b, which reduces or eliminates any electrical conductivity ofthe velvet 42 a and/or 42 b. A coating, such as a Parylene coating,improves fiber retention, but most importantly reduces the electricalconductivity of loose individual fibers, whereby detached fibers wouldnot fall onto the printed circuit board 15 and short circuit anyelectrical circuitry.

Another limitation of the MSA-specified heat sink 8 is that one heatsink can only be applied to one pluggable module 1, i.e. one heat sink 8cannot be used to cool multiple pluggable modules 1. This is due to thefloating nature of the MSA-specified design. When attached to a singlepluggable optic module 1, the heat sink spring clip 9 can account forany tolerance mismatch and maintain contact between the heat sink 8 andthe pluggable module 1. But when additional pluggable modules 1 areadded, it is impossible to contact all of the surfaces due to standardtolerance variation.

With reference to FIGS. 6 and 7, the use of brushes or velvets 42 a andor 42 b, e.g. carbon fiber nanotubes, eliminates the need for the heatsink 8 to move because the individual fibers accommodate the variationsin the surfaces of the heat sink and the optical modules 11. Therefore,a plurality of pluggable optic cages 13 can be mounted on a singleprinted circuit board 56, with a combined electrical connector 57 forconnection to a host device (not shown). Accordingly, only a singlestationary heat sink 58 can be used to dissipate heat from each andevery one of a plurality of optical modules 11 received within the cages13. One or both of the velvets 42 a and/or 42 b is provided for eachmodule 11, either mounted on the heat sink 58 or on each module 11 orboth. The heat sink 58 can cover just the area above the cages 13 or itcan cover, and provide protection and heat dissipation, for the entireprinted circuit board 56.

In the illustrated multi-unit embodiment of FIGS. 6 and 7, the heat sink58 includes a front wall 61 including a plurality of apertures 62providing access to the openings 14, and a rear wall 63 including anaccess port 64 through which the combined electrical connector 57extends. Side walls 66 and 67, preferably include an array of openings,enabling air to circulate through the side walls and over the electricalelements on the printed circuit board 56. The upper wall 68 of the heatsink 58 includes a series of fins or fingers 69 in the area over top ofthe cages 13, i.e. velvets 42 a, for increased heat dissipation.Additional vent openings and/or heat dissipating fins or fingers canalso be provided over top of the other sections of the printed circuitboard 56, as required by their thermal dissipation needs, such asrequired for any processors, FPGA's and memory chips provided in themulti-unit module.

Some pluggable optic modules are not designed for heat sinks. In thesecases, the pluggable optic module is inserted into a cage on the PCBA.There is a gap between the pluggable module and the cage that inhibitsthe flow of heat. Placing carbon fiber nanotube velvet between thepluggable optic module and the cage will create thermal contact betweenthe parts and promote heat flow. This can be accomplished by attachmentof the velvet to both or either of the optical module and the cage.

Accordingly, in another embodiment of the invention, illustrated inFIGS. 8 and 9, an optical module 81, e.g. SFP, is insertable into a cage83, which is mounted on a printed circuit board 85 including anelectrical connector 86. A velvet 82 is mounted directly on the uppersurface of a pluggable optical module 81 or on the inside surface of theupper wall of the cage 83, so that the velvet 82 extends between theoptical module 81 and the cage 83, i.e. across the gap therebetween. Aheat sink 84 is mounted on the outer surface of the upper wall of thecage 83, whereby heat is conducted from the optical module 71 throughthe velvet 82, through the upper wall of the cage 83 to the heat sink84. A second velvet or a conventional thermally conductive material 88,e.g. gel or pad, can be added between the cage outer surface of theupper wall of the cage 83 and the heat sink 84 to enhance thermalconductivity. Accordingly, the heat sink assembly includes The materialsand dimensions of the velvet 82 are the same as those of the velvet 42,relative to the gap between the optical module 71 and the cage 83, e.g.preferably 0.1.2× to 2.0× the gap, more preferably 1.5× to 2.0× the gap,and most preferably 1.66× the gap. Ideally, the velvet 82 covers over25%, preferably greater than 50%, and more preferably greater than 75%of the upper surface of the optical module 81 or the inside surface ofthe upper wall of the cage 83.

We claim:
 1. A cage assembly mountable on a printed circuit board forreceiving an optical module comprising: a cage for slidably receivingthe optical module; an electrical connector mountable on the printedcircuit board for electrically connecting the optical module to theprinted circuit board; and a heat sink assembly mounted on the cage fordissipating heat from the optical module, the heat sink assemblycomprising: a thermally conductive heat sink separated from the opticalmodule by a gap; and a first thermal interface mounted on an undersideof the heat sink or of an upper wall of the cage, including thermallyconductive fibers extending across the gap into the cage for contactingthe optical module.
 2. The cage assembly according to claim 1, whereinthe first thermal interface comprises a velvet of carbon nanotubes. 3.The cage assembly according to claim 1, further comprising an insulatingcoating on the thermally conductive fibers for reducing electricalconductivity thereof.
 4. The cage assembly according to claim 1, whereinthe thermally conductive fibers have a length between 1.5× and 2.0× awidth of the gap.
 5. The cage assembly according to claim 1, whereineach thermally conductive fiber is between 3 and 12 um in diameter. 6.The cage assembly according to claim 1, wherein the thermally conductivefibers have a packing density of from 3% to 15%.
 7. The cage assemblyaccording to claim 1, wherein the first thermal interface has a thermalconductivity of between 1000 and 10,000 W/m²K.
 8. The cage assemblyaccording to claim 1, wherein the cage includes an opening in an upperwall with an area of at least 50% of the area of the upper wall; andwherein the first thermal interface is mounted on the underside of theheat sink, with the thermally conductive fibers extending through theopening into contact with the optical module.
 9. The cage assemblyaccording to claim 1, wherein the first thermal interface is mounted onan inside surface of an upper wall of the cage, with the thermallyconductive fibers extending into contact with the optical module. 10.The cage assembly according to claim 1, further comprising: a pluralityof additional cages for receiving a plurality of additional opticalmodules, respectively, each of the additional optical modules forming arespective gap with the heat sink; a plurality of additional electricalconnectors mounted on the printed circuit board for electricallyconnecting the additional optical modules to the printed circuit board;wherein the heat sink assembly also includes a plurality of additionalfirst thermal interfaces mounted on the underside of the heat sink, eachone of the first thermal interfaces including thermally conductivefibers for extending across the respective gap into the respectiveadditional cage for contacting the respective optical module.
 11. Anoptical module for sliding into a cage assembly, which includes a cage,a first electrical connector, and a heat sink assembly mounted on thecage, the optical module comprising: a housing defining a gap with theheat sink assembly when inserted in the cage; optical and electricalcomponents disposed in the housing for converting optical signals intoelectrical signals and electrical signals into optical signals; a secondelectrical connector extending from the housing for connection to thefirst electrical connector; an optical connector extending from thehousing; and a second thermal interface mounted on the housing includingthermally conductive fibers for extending across the gap into contactwith the heat sink assembly or the cage for dissipating heat from thehousing.
 12. The optical module according to claim 11, wherein thesecond thermal interface comprises a velvet of carbon nanotubes.
 13. Theoptical module according to claim 11, further comprising an insulatingcoating on the thermally conductive fibers for reducing electricalconductivity thereof.
 14. The optical module according to claim 11,wherein the thermally conductive fibers have a length between 1.5× and2.0× a width of the gap between the housing and the heat sink.
 15. Theoptical module according to claim 11, wherein each thermally conductivefiber is between 3 and 12 um in diameter.
 16. The optical moduleaccording to claim 11, wherein the thermally conductive fibers have apacking density of from 3% to 15%.
 17. The optical module according toclaim 11, wherein the second thermal interface has a thermalconductivity of between 1000 and 10,000 W/m²K.
 18. The optical moduleaccording to claim 11, wherein the cage includes an opening in an upperwall with an area of at least 50% of the area of the upper wall; andwherein the first thermal interface extends through the opening intocontact with the heat sink assembly.
 19. The optical module according toclaim 11, wherein the thermally conductive fibers extending into contactwith the cage.
 20. An optical system including: the cage assemblydefined in claim 1; and the optical module defined in claim 11; whereinthe thermally conductive fibers from each of the first and secondthermal interfaces have a length between 0.6× and 1.0× a width of thegap between the optical module and the heat sink for engaging eachother.